This invention relates to a method and system for providing circuit-switched to IP Multimedia Subsystem voice call continuity with a single radio. While the invention is particularly directed to the art of telecommunications, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications.
By way of background, wireless telecommunication networks, which are well known, allow mobile devices to communicate with each other and other networks, such as the Internet and the Public Switched Telephone Network (PSTN). In general, the Universal Mobile Telecommunication System (UMTS) is a third generation wireless communications system that has evolved from the Global System for Mobile communication (GSM). The UMTS is intended to provide various mobile communication services by combining a UMTS Terrestrial Radio Access Network (UTRAN) with a Circuit-Switched Core Network (CS CN) based on the GSM and a Packet-Switched Core Network (PS CN) providing General Packet Radio Service (GPRS). The specification of the UMTS is currently being developed by a standardization group called the Third Generation Partnership Project (3GPP).
The IP Multimedia Subsystem (IMS) is an architectural framework for delivering internet protocol (IP) multimedia to mobile users via any IP access network providing Packet-Switched (PS) services. It was originally designed by 3GPP, and it is part of the vision for evolving mobile networks beyond GSM. To ease the integration with the Internet, IMS generally uses IETF (i.e., Internet) protocols such as Session Initiation Protocol (SIP). According to 3GPP, IMS is intended to aid the access of multimedia and voice applications from both wireless and wireline terminals, that is, to aid a form of fixed mobile convergence. This is done by having a horizontal control layer that isolates the access network from the service layer. Services need not have their own control functions, as the control layer is a common horizontal layer.
LTE/SAE (Long Term Evolution/System Architecture Evolution) is the name given to a project within 3GPP to improve the UMTS mobile phone standard to cope with future requirements. Goals include improving efficiency, lowering costs, improving services, making use of new spectrum opportunities, and better integration with other open standards. LTE/SAE's radio access is called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and is intended to support only packet-switched services. The LTE/SAE PS CN offers PS services via a variety of access networks, such as GSM's/UMTS's own radio network (GERAN, UTRAN, and E-UTRAN), WiFi and even “competing” systems such as CDMA2000 and WiMax. Users of non-UMTS radio networks would be provided with an entry-point into the IP network, with different levels of security depending on the trustworthiness of the network being used to make the connection. Users of GSM/UMTS networks would use an integrated system where all authentication at every level of the system is covered by a single system, while users accessing the UMTS network via WiMAX and other similar technologies would handle the WiMAX connection one way (for example, authenticating themselves via a MAC or ESN address) and the UMTS link-up another way.
3GPP TS 23.206 defines a feature called Voice Call Continuity (VCC) that provides the capability to transfer the path of an existing voice call between a 3GPP circuit-switched (CS) system (GSM/UMTS) and IMS via packet-switched (PS) access network, and vice versa, to enable the continuation of a voice call when only one of two types of access networks are available.
Transfer of the path from IMS to CS is realized by placing a CS call to IMS that takes over for an existing IMS call. Transfer of the path from CS to IMS is realized by placing an IMS call that takes over for an existing CS call.
Current procedures in 3GPP TS 23.206 assume that it is possible for the user equipment (UE) to transmit and receive simultaneously in both IMS via PS access and CS access while transferring the media path to minimize disruption of speech and/or other media. Without simultaneous transmit and receive capability in both IMS and CS, the media disruption may last several seconds and be unacceptably long.
Simultaneous transmit and receive capability is available with a single radio for IMS over the UMTS packet-switched (PS) domain and for CS over the UMTS circuit-switched (CS) domain. Simultaneous transmit and receive capability requires two radios for IMS over WiFi and CS over GSM or UMTS. This is an acceptable burden for dual-mode WiFi and GSM/UMTS UEs, since these radios do not share chip sets.
Simultaneous transmit and receive capability for IMS over LTE and CS over GSM or UMTS also requires two radios, but this is an unacceptable burden on these devices for reasons of cost, form factor, etc., since these radios will be available in common chip sets.
Thus, the issue is how to support voice call continuity between IMS over LTE and CS over GSM/UMTS with an acceptably short media disruption during transfer of the media path using only a single radio in the UE.
Section 7.19.1 of 3GPP TS 23.882 includes a discussion of the major options currently available. However, the underlying problem with all solutions proposed so far is that there is likely to be an unacceptable disruption in media during the transfer of the media path. Note that this disclosure only focuses on the transfer of the path of a voice call from IMS over LTE to UMTS CS (GSM/UMTS). Other solutions may apply to the transfer from CS to IMS, but they are outside the scope of this disclosure.
The ideal solution requires that sufficient information be exchanged between the UE and the network prior to transfer of the media path so that the radio switch from LTE to GSM/UMTS occurs at the same time that the media path is transferred between the networks. If either occurs significantly before the other, there will be a corresponding disruption in the media. This coordination is very difficult. Arguably, the best proposed solution to date initiates the transfer of the media path just before performing an inter-system handover procedure that results in the change of radio mode. Unfortunately, the inter-system handover procedure may take as much as a few seconds to perform in a typical network, leading to a potentially significant disruption in speech or other media.
Thus, the present invention contemplates a new and improved method (and system) that resolves the above-referenced difficulties and others.